1. Field of the Invention
This invention relates to the planarization of semiconductor devices by Chemical-Mechanical-Polishing (CMP) techniques and, more particularly, to the formulation of polishing slurries for metals including tungsten wherein the slurry has a prolonged stability against settling of abrasive material and which is less corrosive to equipment.
2. Description of the Prior Art
Chemical-Mechanical-Polishing techniques are used in the semiconductor processing industry to enable interconnect metallurgy of the highest density to be fabricated. Based on the teachings in U.S. Pat. No. 4,789,648 to Chow et al. and U.S. Pat. No. 4,944,836 to Beyer et al., CMP has enabled the continued shrinking of semiconductor device dimensions by the practical and manufacturable introduction of planarization of both metals and interlevel dielectrics to semiconductor processing. By maintaining the surface of a semiconductor device as flat as possible, the rendering of optical images of ever smaller size continues to be possible.
CMP generally is based on the principles that most hard materials are reactive with oxidants to form oxides and other softer compounds which, in turn, can be polished away. If the process of reacting and polishing can be controlled sufficiently, semiconductor metallurgy will continue to lead the path to miniaturization.
It is known to planarize tungsten metallurgy by polishing with an aqueous slurry comprising alumina in ferric nitrate. See "PROCESS OPTIMIZATION OF TUNGSTEN CMP," V. Blaschke and K. Holland, Chemical Mechanical Polishing--Metals Seminar (CMP), SEMICON/Southwest '95; Austin, Tex. Oct. 23, 1995. This slurry, when used in an IPEC-Westech Systems wafer polisher at a feedrate of about 125 ml per minute, has the capability of polishing tungsten at 3,000 Angstroms per minute. The slurry, however, has the following disadvantages:
1. The alumina colloidal suspension is unstable and the ferric nitrate solution must be agitated constantly to keep the suspension dispersed in the solution. If left alone, the suspension collapses within a few hours rendering it impossible to formulate batches of slurry in advance. The settling of the alumina particles indicates that the particles are not well dispersed at all, a condition which could easily lead to scratching and cause other polishing irregularities since the slurry actually contains particulates and aggomerates large enough to settle out of suspension. This also can cause Foreign Material (FM) problems and loss of product yield.
2. The polishing slurry leaves rust residue on items throughout the CMP tool area. Whenever some slurry is spilled or sprayed, orange rust is left behind when the slurry dries. The stains are an indication that the ferric ion is prone to polymerizing to oxo-bridged species, leading to ferric-bearing oxide residues. These residues are a source of FM. The polymerization can also account for agglomeration of the alumina, as the polymerized ferric species likely promotes adhesion of alumina particles to each other. This condition often causes tools to be shut down for cleaning and removal of slurry cakes by the use of a hammer.
3. The heterogenity of the slurry also prohibits the bulk feeding of the slurry to the tools. Because the particulates in the slurry have a tendency to settle out, the particulates will precipitate out within any delivery system clogging tubing and valves.
4. The tendency of the ferric components to form rust residue causes corrosion wherever the slurry lands and dries on stainless steel parts of the polishing tools. This is a cause of FM problems and eventually destroys polisher components.
Other ferric-based salts have been proposed and include ferricyanides as oxidizers, combined with an acetate buffer and acetic acid using a silica abrasive as described in U.S. Pat. No. 5,407,526 to Danielson et al. and U.S. Pat. No. 5,516,346 to Cadien etal.
U.S. Pat. No. 5,527,423 to Neville et al. describes a polishing slurry for polishing metals. The slurry includes ferric nitrate nonahydrate and deionized water in which a special fumed alumina or silica is used to provide stability of particles in suspension. The disclosure also suggests other additives and stabilizers which may be added immediately prior to use. The differences in polish rates obtained are attributed to the high surface area fumed silica or alumina of the invention and the resulting slurry suitable for most metals.